The first involved collecting and examining hundreds of photographs and moments of video looking for a suspicious person or persons in a sea of thousands. But a second investigation was underway simultaneously.

If the odds of identifying a suspect in the crowd was like finding a needle in a haystack, then finding a likely suspect amid the tens of thousands of suspects kept on FBI watch lists and monitored online for suspicious activity was like "finding a needle in 1,000 haystacks," said one federal law enforcement source.

The FBI and other agencies keep an eye on thousands of extremist websites on which tens of thousands of individuals are communicating, sending millions of messages between them. Difficult as it sounds, monitoring such networks is crucial to predicting and preventing attacks similar to Boston, in which three people were killed and at least 200 were injured, current and former federal agents told ABCNews.com.

Based on information gleaned from online chatter, the FBI has apprehended suspects directly or engaged them in sting operations that officials claim led to foiling a suicide attack on the Capitol, a plot to bomb synagogues in New York, and another to shoot stinger missiles at airplanes.

Tamerlan Tsarnaev, 26, the older of the two brothers suspected of carrying out last week's attack in Boston, was known to federal investigators and flagged by Russian authorities as a possible threat in 2011. The Russians suspected Tsarnaev of interacting with known terrorist groups or individuals by monitoring electronic conversations, a federal source told ABC News.

American authorities cleared Tsarnaev and failed to continue to seriously monitor him, a source said.

Sen. Lindsay Graham, R- S.C., on Monday said the FBI should have better monitored Tsarnaev online, watching his behavior on "radical websites" to realize he was going "down a dangerous path."

Federal agents are redoubling their efforts to comb through an extensive network of extremists, separating legitimately dangerous threats from online lurkers and sympathizers.

"There are too many of these guys to monitor them all individually," Garrett said. "And they go to more than just one website."

Investigators are looking for clues and connections online.

"Something has to pop on the network," Garrett said

"Are they just talking?" he added. "Or are they talking to anyone who we already know is a really bad guy? Are they meeting up with other people who we're monitoring? These are the questions we need to be asking moving forward."